In order to enhance the durability, riding comfort, and braking power of a tire equipped in a vehicle, a pressure and temperature of the tire should be maintained in an appropriate state. To this end, an apparatus for monitoring pressures and temperatures of tires is needed. In automotive engineering, the apparatus is called a Tire Pressure Monitoring System (TPMS).
FIG. 1 is a block diagram illustrating a configuration of a TPMS sensor included in a general TPMS.
Referring to FIG. 1, the TPMS sensor includes a pressure sensor 11, a temperature sensor 13, an acceleration sensor 15, a supply voltage sensor 17, a controller 19, a Radio Frequency (RF) transmitter 21, and a Low Frequency (LF) receiver 23.
The pressure sensor 11 senses an internal pressure of a tire to detect a low pressure and a pressure change. The temperature sensor 13 senses an internal temperature of the tire to correct an error of the pressure sensor, and senses a tire temperature. The acceleration sensor 15 senses an acceleration and a speed to determine whether a vehicle is running and detect a sensor position. The supply voltage sensor 17 measures a voltage of a battery 25, which is provided in the TPMS sensor, to detect a low pressure state of the battery 25.
When the TPMS sensor periodically senses a tire pressure and a tire temperature in running, the TPMS sensor periodically performs RF transmission. In an operation of periodically sensing the tire pressure, the TPMS sensor compares a currently sensed pressure and a previously sensed pressure, and when a pressure change equal to a certain pressure or more occurs, the TPMS sensor performs RF transmission based on an internal pressure change of a tire. When a response command of the TPMS sensor is requested by the outside for the special purposes of vehicle maintenance, a production line, and determination of a sensor position by using an LF initiator, the TPMS sensor performs RF transmission based on a signal of an external device so as to respond to the requested command.
As described above, the TPMS sensor transmits RF data when an internal pressure of a tire is changed. Examples of an internal pressure change of the tire includes a pressure change, which occurs due to a change in an internal air amount of the tire, and a pressure change which occurs due to a change in an internal air temperature of the tire.
In particular, when an internal air temperature of a tire is changed, a pressure change based on an ideal gas equation expressed as the following Equation (1) occurs.
                              P          warm                =                                                            (                                  RCP                  +                                      P                    ambient                                                  )                            ·                              T                tire                                                    T              reference                                -                      P            ambient                                              (        1        )            where Pwarm denotes an internal pressure of a tire based on an internal temperature change of the tire, RCP denotes a recommend cold pressure proposed by a vehicle manufacturer, Ttire denotes an internal temperature of the tire, Treference denotes an external temperature and external temperature setting value of a vehicle, and Pambient denotes an atmospheric pressure.
An internal air temperature of a tire is changed when an external temperature of the tire is changed or when a wheel temperature increases due to braking of a vehicle or a friction of the tire and a road surface in running of the vehicle. Considering that an external air temperature is very slowly changed, a difference between an internal air temperature and an external air temperature of a tire occurs due to a temperature change caused by running and braking of a vehicle.
Domestic and foreign regulations associated with the TPMS regulate conditions in which a relative tire pressure Pwarm is calculated depending on a tire temperature, and a low pressure warning and a release of the warning are performed based on the calculated tire pressure.
In Equation (1), RCP is set by a vehicle manufacturer, and Ttire is a value that is calculated by using a tire temperature sensed by the TPMS sensor. However, Pambient and Treference should be calculated based on vehicle information or information estimated from other sensing data. Treference is an external temperature, and a TPMS receiver communicating with the TPMS sensor may obtain Treference from another electronic control device through vehicle network communication such as a Controller Area Network (CAN).
A vehicle, which is not equipped with a device for measuring an external temperature, measures a vehicle stop time to estimate the external temperature. In this case, the TPMS sensor receives a result, which is obtained by monitoring a vehicle stop state, from another electronic control device through the TPMS receiver.
A vehicle stop state may be monitored by turning on/off an ignition. When the ignition is turned off, an electronic control device for monitoring the vehicle stop state causes a waste of power of a vehicle battery because using a battery power of a vehicle.
In regulations, a vehicle stop time is set to one hour. Therefore, when an internal temperature of a tire is changed identically to an external temperature within one hour, an error occurs in estimating an external temperature. Generally, an internal temperature of a tire increases due to running and braking of the tire, and when braking of a vehicle is continuously maintained for one hour, an error of about 2 to 3 degrees C. occurs between an internal temperature of the tire and an external temperature. The error causes an error margin of a setting value when a warning notice for notifying a low pressure state of a tire is set.